Escherichia coli, also known in shorter form as E. coli, are prokaryotic, and therefore they are smaller than the eukaryotic cells in humans, and also do not have membrane-enclosed organelles. The phylum in which they are classified, proteobacteria, is the largest group of bacteria in terms of the number of described species in which it contains. In addition to the single chromosome that all bacteria have, E. coli also contain plasmids, which are approximately 2000 to 6000 base pairs long.
Relationship to Humans Most strands of E. coli are harmless to humans, and several types even exist normally in the human gut in a symbiotic relationship with the host by absorbing nutrients and in turn producing vitamin K2 and fighting against pathogenic, or harmful, bacteria (15). Pathogenic strains release endotoxins when they bacteria lyses, causing fever, vomiting, and diarrhea in humans. The relatively new field of biotechnology has extensive use for the bacteria. An early experiment in 1970s conducted by Stanley Cohen and Herbert Boyer with E. coli plasmids began the era of recombinant DNA, the science of combining pieces of DNA from multiple organisms to create a new genome. By inserting the modified gene of a eukaryote, E. coli and other prokaryotes can produce useful natural products like human insulin. Also, the restriction enzyme EcoRI is used by humans prior to gel electrophoresis to cut the sample DNA into fragments for the test. E. coli has also been used by scientists to clean up nuclear wastes. It was recently discovered that the organism has the ability to break down phytic acid and release phosphate molecules, which bind to uranium (12). However, only nonpathogenic strains of E. coli is used in biotechnology.
Recombinant DNA Technology Using EcoRI
Habitat and Niche E. coli thrive in the gastrointestinal tract of humans and other mammals because it is a warm, moist, and nutrient rich habitat (4). It is the most abundant facultatively anaerobic microorganism found in the intestines of mammals. The organism commonly enters its host by contaminated food sources, such as vegetation.
Predator Avoidance The only predators of E. coli are bacteriophages, or viruses that target bacteria, and their human hosts. Viruses use cells to produce viral copies of itself, starting by inserting its own genetic material into that of the target. E. coli have a restriction enzyme known as EcoRI, which recognizes particular DNA sequences injected by bacteriophages sequences and cuts it out to protect the cell from being overtaken. When pathogenic strains of E. coli target their larger predator, they release endotoxins, which are lipopolysacchrides that form part of the outer bacterial membrane, when they lyse. These chemicals vary by strain, commonly causing fever, vomiting, diarrhea, or any combination of the three, but rarely cause death. E.coli have become resistant to antibiotics due to overuse, which kills off the vulnerable strains and gives rise to those that are not effected by such medications. Mutations in the genetic information of the organism allow it to possibly and unintentionally change in a way that gives it antibiotic resistance, and may then undergo conjugation, in which genes in a plasmid are transferred from one cell to another. Antibacterial resistance and other defensive genes are often passed this way in order to maintain the species.
Nutrient Acquisition As a single celled organism, E. coli have three methods of acquiring nutrition: passive transport via diffusion across the plasma membrane, facilitated diffusion via protein channels, and active transport via proteins and expenditure of cellular energy. Diffusion limits the cell's intake to a few molecules due to the selective nature of the cytoplasmic membrane, which allows only small, uncharged polar molecules and gases through. In order for nutrients to be taken in, the concentration outside of the cell must be greater than that inside, so that the molecules flow into the cell where concentration is lower. Just like diffusion, facilitated diffusion also doesn't consume energy from the cell. Carrier proteins, sometimes called uniporters, and porin proteins assist molecules that cannot diffuse through the plasma membrane to enter; for example, the carrier protein GlpF permease, called the GlpF glycerol MIP channel, transports glycerol across the plasma membrane in E. coli. Molecules are able to cross the membrane at a faster rate than diffusion without requiring energy with facilitated diffusion. Surrounded by two selectively permeable barriers, two types of porin proteins facilitate the transport of molecules from the outside of the cell to inside the periplasmic space. Nonspecific porins allow a wide range of small, hydrophobic and hydrophilic molecules to pass through the membrane, but large molecules cannot pass through these porins because of their small size. On the other hand, substrate specific porins recognize specific types of molecules and allow them to pass through. Most of these molecules are too large to enter E. coli cells through the nonspecific porins. Some of the substrate specific porins will recognize a similar family of molecules. The third method of nutrient acquisition is active transport with proteins and the use of cellular energy. ATP (the energy transporting molecules of cells) Binding Cassette transporters, known also as ABC transporters, are one way that molecules are actively transported into the cell, and are made up of a solute binding protein, a transmembrane spanning protein, and an ATP hydrolyzing protein. The hydrolysis--or the splitting of a molecule via a reaction with water--of ATP is the driving force of the uptake of molecules. Another type is P-Type ATPases, which are proteins that transport cations like K+ and Mg+2, also using ATP as the source of energy. Secondary transport systems uptake many amino acids, inorganic molecules, and sugars. Group translocation systems are more complex than other systems, as they chemically modify molecules upon entry, using either ATP or PEP (phosphoeonolpyruvate, another energy carrying molecule) as energy sources (2).
Reproduction and Life Cycle Matured individual E. coli reproduce asexually via binary fission. Each individual has a single circular chromosome that must be replicated, so two replication forks depart from the ori (origin of replication) sequence. Even though it takes 40 minutes to replicate an entire chromosome, E. coli daughter chromosomes begin replication again prior to completion, and therefore are able to divide approximately every 20 minutes. All offspring are genetically identical to the parent, and thusly are clones, so any changes in the genetic coding between generations are due to mutations during chromosome replication. Only prokaryotes may undergo binary fission, and it begins with DNA replication at the ori sequence in the centre of the cell. Secondly, the cell replicates its DNA as it grows, followed by the daughter chromosomes separating, led by the regions including the ori. Finally the cells divide by pinching in at the middle and cytokinesis--the division of cells--is complete.
Growth and Development E. coli develop on the mucus membranes of the epithelium of the human intestines, growing steadily until enough nutrients are available to undergo mitosis. Under optimal conditions, the population grows at a predictable rate shown in the model below. The "lag" phase regards the slow development of the cell population as the individuals are still acclimating to the new conditions and are simply growing in size. During the "exponential" or log phase, cells are dividing regularly, causing the population to grow in a geometric progression. When the population reaches its carrying capacity, it reaches the "stationary" phase; growth stagnates due to the exhaustion of available nutrients, the accumulation of inhibitory metabolites, and the lack of physical space.
Populations Grow at Predicable Rates Under Optimal Conditions
Integument The cell of the E. coli is protected by two layers: the capsid, and the cell wall beneath it. The capsid is a slimy layer composed mostly of polysaccharides that protects it from white blood cells and drying out, as well as help the organism attach to other cells. However, it is not essential to E. coli, so an individual can survive even if it were to lose it. A cell wall supports the cell and determines its shape. It contains peptidoglycan, a polymer of amino sugars that are cross linked by covalent bonds to peptide, which forms a single giant molecule that surrounds the entire cell. Gram positive E. coli have cell membranes and and thick walls of peptidoglycan, whereas their gram negative counterparts have cell membranes, a thinner layer of peptidoglycan, as well as an outer layer of lipopolysaccharides. These lipopolysaccharides make gram negative E. coli less susceptible to antibiotics, and thus more threatening to humans should they be pathogenic (5).
Movement Motile E. coli propels itself forward by rotating its flagella counterclockwise. Periodically, it will stop, tumble, and continue moving in a new, random direction, as seen in part "A" of the diagram. Chemotaxi is the movement toward or away from chemicals. As seen in part "B," E. coli will travel toward substances such as amino acids and sugars (1).
Model of the Movement and Chemotaxi of E. coli
(11)
Sensing the Environment E. coli respond to changes in the environment by changing the expression of their genes. Individuals can sense either the presence of absence of gasses and chemicals in its environment, and swims towards or away from what it senses as the sources. An individual can stop swimming and grow fimbriae that specifically attach to a cell or a receptor on the surface of the cell. When mainly lactose is present, the cell produces three lactose-specific enzymes that transport the sugar into the cell and breaks it down; when the environment primarily contains glucose, very few of those enzymes are present. The coding for the lactose metabolizing enzymes are adjacent to each other on the E. coli chromosome, and therefore are all created at once. Repressor proteins known as lac operons prevent transcription until the lac-encoded proteins are needed--when lactose is the predominant sugar available in the environment. Either all or none of these enzymes are made at any particular time.
Gas Exchange Anaerobic respiration is used by E. coli to perform gas exchange and is done through the membrane of the cell.
Waste Removal Because E. Coli is a unicellular prokaryote, waste is removed by the semipermeable plasma membrane (14).
Environmental Physiology E.coli thrives in an environment with a neutral pH level and a stable temperature around 37ยบ Celsius; however it does possess the adaptive qualities necessary to survive in an environment with changing temperatures. Utilizing its peritrichous flagella, or flagella that cover the body surface in a uniform distribution, E.coli move away from specific areas where it senses chemical imbalances and changes in osmolarity. E.coli also physically change the diameter of porins found on the cell membrane in response to changes in temperature and concentration, allowing larger molecules to pass through if necessary by expanding, and contracting to prevent permeation by undesired molecules. (10)
Internal Circulation E.coli does not have a circulatory system, for it only has a single cell. Nutrients are transported through the cytoplasm directly to where they are used.
Chemical Control E.coli is a single cellular organism and therefore does not have a chemical control system such as an endocrine system. Changes in the organism are a direct result of gene expression of DNA.
Review Questions 1. How does E. coli grow and develop? What are the three different stages that are involved? 2. What makes E.coli easy to use by biologists? 3. How can E. coli defend against predators? How does this complicate human protection against E. coli?
Aynslie D'Avanzo November-December 2013
Classification/Diagnostic Characteristics
Classification (10):
- Domain: Bacteria
- Kingdom: Bacteria
- Phylum: Proteobacteria
- Class: Gamma Proteobacteria
- Order: Enterobacteriales
- Family: Enterobacteriaceae
- Genus: Escherichia
- Species: Escherichia coli (E. coli)
Escherichia coli, also known in shorter form as E. coli, are prokaryotic, and therefore they are smaller than the eukaryotic cells in humans, and also do not have membrane-enclosed organelles. The phylum in which they are classified, proteobacteria, is the largest group of bacteria in terms of the number of described species in which it contains. In addition to the single chromosome that all bacteria have, E. coli also contain plasmids, which are approximately 2000 to 6000 base pairs long.Relationship to Humans
Most strands of E. coli are harmless to humans, and several types even exist normally in the human gut in a symbiotic relationship with the host by absorbing nutrients and in turn producing vitamin K2 and fighting against pathogenic, or harmful, bacteria (15). Pathogenic strains release endotoxins when they bacteria lyses, causing fever, vomiting, and diarrhea in humans.
The relatively new field of biotechnology has extensive use for the bacteria. An early experiment in 1970s conducted by Stanley Cohen and Herbert Boyer with E. coli plasmids began the era of recombinant DNA, the science of combining pieces of DNA from multiple organisms to create a new genome. By inserting the modified gene of a eukaryote, E. coli and other prokaryotes can produce useful natural products like human insulin. Also, the restriction enzyme EcoRI is used by humans prior to gel electrophoresis to cut the sample DNA into fragments for the test. E. coli has also been used by scientists to clean up nuclear wastes. It was recently discovered that the organism has the ability to break down phytic acid and release phosphate molecules, which bind to uranium (12). However, only nonpathogenic strains of E. coli is used in biotechnology.
Habitat and Niche
E. coli thrive in the gastrointestinal tract of humans and other mammals because it is a warm, moist, and nutrient rich habitat (4). It is the most abundant facultatively anaerobic microorganism found in the intestines of mammals. The organism commonly enters its host by contaminated food sources, such as vegetation.
Predator Avoidance
The only predators of E. coli are bacteriophages, or viruses that target bacteria, and their human hosts. Viruses use cells to produce viral copies of itself, starting by inserting its own genetic material into that of the target. E. coli have a restriction enzyme known as EcoRI, which recognizes particular DNA sequences injected by bacteriophages sequences and cuts it out to protect the cell from being overtaken. When pathogenic strains of E. coli target their larger predator, they release endotoxins, which are lipopolysacchrides that form part of the outer bacterial membrane, when they lyse. These chemicals vary by strain, commonly causing fever, vomiting, diarrhea, or any combination of the three, but rarely cause death. E.coli have become resistant to antibiotics due to overuse, which kills off the vulnerable strains and gives rise to those that are not effected by such medications. Mutations in the genetic information of the organism allow it to possibly and unintentionally change in a way that gives it antibiotic resistance, and may then undergo conjugation, in which genes in a plasmid are transferred from one cell to another. Antibacterial resistance and other defensive genes are often passed this way in order to maintain the species.
Nutrient Acquisition
As a single celled organism, E. coli have three methods of acquiring nutrition: passive transport via diffusion across the plasma membrane, facilitated diffusion via protein channels, and active transport via proteins and expenditure of cellular energy. Diffusion limits the cell's intake to a few molecules due to the selective nature of the cytoplasmic membrane, which allows only small, uncharged polar molecules and gases through. In order for nutrients to be taken in, the concentration outside of the cell must be greater than that inside, so that the molecules flow into the cell where concentration is lower. Just like diffusion, facilitated diffusion also doesn't consume energy from the cell. Carrier proteins, sometimes called uniporters, and porin proteins assist molecules that cannot diffuse through the plasma membrane to enter; for example, the carrier protein GlpF permease, called the GlpF glycerol MIP channel, transports glycerol across the plasma membrane in E. coli. Molecules are able to cross the membrane at a faster rate than diffusion without requiring energy with facilitated diffusion. Surrounded by two selectively permeable barriers, two types of porin proteins facilitate the transport of molecules from the outside of the cell to inside the periplasmic space. Nonspecific porins allow a wide range of small, hydrophobic and hydrophilic molecules to pass through the membrane, but large molecules cannot pass through these porins because of their small size. On the other hand, substrate specific porins recognize specific types of molecules and allow them to pass through. Most of these molecules are too large to enter E. coli cells through the nonspecific porins. Some of the substrate specific porins will recognize a similar family of molecules. The third method of nutrient acquisition is active transport with proteins and the use of cellular energy. ATP (the energy transporting molecules of cells) Binding Cassette transporters, known also as ABC transporters, are one way that molecules are actively transported into the cell, and are made up of a solute binding protein, a transmembrane spanning protein, and an ATP hydrolyzing protein. The hydrolysis--or the splitting of a molecule via a reaction with water--of ATP is the driving force of the uptake of molecules. Another type is P-Type ATPases, which are proteins that transport cations like K+ and Mg+2, also using ATP as the source of energy. Secondary transport systems uptake many amino acids, inorganic molecules, and sugars. Group translocation systems are more complex than other systems, as they chemically modify molecules upon entry, using either ATP or PEP (phosphoeonolpyruvate, another energy carrying molecule) as energy sources (2).
Reproduction and Life Cycle
Matured individual E. coli reproduce asexually via binary fission. Each individual has a single circular chromosome that must be replicated, so two replication forks depart from the ori (origin of replication) sequence. Even though it takes 40 minutes to replicate an entire chromosome, E. coli daughter chromosomes begin replication again prior to completion, and therefore are able to divide approximately every 20 minutes. All offspring are genetically identical to the parent, and thusly are clones, so any changes in the genetic coding between generations are due to mutations during chromosome replication. Only prokaryotes may undergo binary fission, and it begins with DNA replication at the ori sequence in the centre of the cell. Secondly, the cell replicates its DNA as it grows, followed by the daughter chromosomes separating, led by the regions including the ori. Finally the cells divide by pinching in at the middle and cytokinesis--the division of cells--is complete.
Growth and Development
E. coli develop on the mucus membranes of the epithelium of the human intestines, growing steadily until enough nutrients are available to undergo mitosis. Under optimal conditions, the population grows at a predictable rate shown in the model below. The "lag" phase regards the slow development of the cell population as the individuals are still acclimating to the new conditions and are simply growing in size. During the "exponential" or log phase, cells are dividing regularly, causing the population to grow in a geometric progression. When the population reaches its carrying capacity, it reaches the "stationary" phase; growth stagnates due to the exhaustion of available nutrients, the accumulation of inhibitory metabolites, and the lack of physical space.
Integument
The cell of the E. coli is protected by two layers: the capsid, and the cell wall beneath it. The capsid is a slimy layer composed mostly of polysaccharides that protects it from white blood cells and drying out, as well as help the organism attach to other cells. However, it is not essential to E. coli, so an individual can survive even if it were to lose it. A cell wall supports the cell and determines its shape. It contains peptidoglycan, a polymer of amino sugars that are cross linked by covalent bonds to peptide, which forms a single giant molecule that surrounds the entire cell. Gram positive E. coli have cell membranes and and thick walls of peptidoglycan, whereas their gram negative counterparts have cell membranes, a thinner layer of peptidoglycan, as well as an outer layer of lipopolysaccharides. These lipopolysaccharides make gram negative E. coli less susceptible to antibiotics, and thus more threatening to humans should they be pathogenic (5).
Movement
Motile E. coli propels itself forward by rotating its flagella counterclockwise. Periodically, it will stop, tumble, and continue moving in a new, random direction, as seen in part "A" of the diagram. Chemotaxi is the movement toward or away from chemicals. As seen in part "B," E. coli will travel toward substances such as amino acids and sugars (1).
Sensing the Environment
E. coli respond to changes in the environment by changing the expression of their genes. Individuals can sense either the presence of absence of gasses and chemicals in its environment, and swims towards or away from what it senses as the sources. An individual can stop swimming and grow fimbriae that specifically attach to a cell or a receptor on the surface of the cell. When mainly lactose is present, the cell produces three lactose-specific enzymes that transport the sugar into the cell and breaks it down; when the environment primarily contains glucose, very few of those enzymes are present. The coding for the lactose metabolizing enzymes are adjacent to each other on the E. coli chromosome, and therefore are all created at once. Repressor proteins known as lac operons prevent transcription until the lac-encoded proteins are needed--when lactose is the predominant sugar available in the environment. Either all or none of these enzymes are made at any particular time.
Gas Exchange
Anaerobic respiration is used by E. coli to perform gas exchange and is done through the membrane of the cell.
Waste Removal
Because E. Coli is a unicellular prokaryote, waste is removed by the semipermeable plasma membrane (14).
Environmental Physiology
E.coli thrives in an environment with a neutral pH level and a stable temperature around 37ยบ Celsius; however it does possess the adaptive qualities necessary to survive in an environment with changing temperatures. Utilizing its peritrichous flagella, or flagella that cover the body surface in a uniform distribution, E.coli move away from specific areas where it senses chemical imbalances and changes in osmolarity. E.coli also physically change the diameter of porins found on the cell membrane in response to changes in temperature and concentration, allowing larger molecules to pass through if necessary by expanding, and contracting to prevent permeation by undesired molecules. (10)
Internal Circulation
E.coli does not have a circulatory system, for it only has a single cell. Nutrients are transported through the cytoplasm directly to where they are used.
Chemical Control
E.coli is a single cellular organism and therefore does not have a chemical control system such as an endocrine system. Changes in the organism are a direct result of gene expression of DNA.
Review Questions
1. How does E. coli grow and develop? What are the three different stages that are involved?
2. What makes E.coli easy to use by biologists?
3. How can E. coli defend against predators? How does this complicate human protection against E. coli?
References
1.chemotaxis.biology.utah.edu/Parkinson_Lab/projects/ecolichemotaxis/ecolichemotaxis.html
2. http://ecolistudentportal.org/article_nutrient_transport#_
3. http://bioweb.uwlax.edu/bio203/s2008/moder_just/habitat.htm
4. http://science.howstuffworks.com/life/evolution/evolution4.htm
5. http://openwetware.org/wiki/Dixon%27s_Biotech (Unit 1 Powerpoint)
6.http://www.sciencedaily.com/releases/2012/10/121030210351.htm
7. http://www.expertsmind.com/topic/microbiology/recombinant-microorganisms-in-biotechnology-92494.aspx
8. http://textbookofbacteriology.net/growth_3.html
9. http://www.cellsalive.com/ecoli.htm
10. http://bioweb.uwlax.edu/bio203/s2008/moder_just/adaptation.htm
11. http://www.ccbi.cam.ac.uk/iGEM2005/images/f/f9/Chemotaxis.gif
12. http://inhabitat.com/e-coli-cleans-up-nuclear-waste-cheaply-efficiently/
13. http://bioweb.uwlax.edu/bio203/s2008/moder_just/habitat.htm
14. http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/gncw.html
15. http://www.medicalnewstoday.com/articles/68511.php